This invention relates generally to valve gated injection molding and more particularly to an improved double acting mechanism for actuating the valve pins.
As shown, for instance, in the applicant's U.S. Pat. No. 4,222,733 which issued Sept. 16, 1980, it is known to use a pneumatically driven rocker arm as a valve pin actuator mechanism. However, this type of arrangement requires a separate mechanism and power source for each valve pin. In addition, it is single acting in that it only actuates the valve pin to the closed position and it is opened by the application of melt pressure after the air pressure is released.
The applicant's U.S. Pat. No. 4,212,627 which issued June 15, 1980 shows a valve pin actuating mechanism which uses a single power source to drive a number of valve pins but, once again, is single acting. In attempting to reduce costs and operating problems by using a single power source to drive a number of valve pins, one of the problems is providing for accurate seating of each of the valve pins. If there is no provision for adjustment of the position of each valve pin relative to the others, then they must all be made to extremely close tolerances which substantially increases costs and is a potential source of problems.
While single acting valve pin actuation is satisfactory for molding certain materials under certain conditions, it becomes a problem with more difficult to mold materials, such as glass filled polyester flame retardant material. In a normal single acting mechanism a film of the melt itself acts as a lubricant which allows the melt pressure to open the valve. However, under certain conditions this operation may be impaired by the film of melt being carbonized while the valve is closed. Furthermore, in order to avoid leakage with certain materials, it is necessary to reduce tolerances to the point where melt pressure cannot be relied upon to open the valve.
While double acting valve pin actuators are known, they are not mechanical mechanisms with provision for initial adjustment or calibration to facilitate a number of units being driven by a single power source.